This chapter offers an insight into the European Union’s (EU) participation in the Organisation for Economic Co-Operation and Development (OECD). Exploring historical, institutional and substantive perspectives, it argues that the EU, despite not holding official membership of the OECD, is a de facto Member. The chapter begins by highlighting the strong historical links between the two organisations. It then demonstrates that, from an institutional perspective, the EU has almost the same legal rights as OECD Members and has a strong influence on the decisionmaking process and budget of the organisation, although it does not formally participate therein. Finally, from a substantive perspective, the chapter shows that the OECD standardsetting activity and policymaking tools, such as peer reviews, have an important impact and influence on EU regulatory policies, as the OECD’s areas of work cover almost all EU competences.